Method for removing foreign matter from glass substrate surface and method for processing glass substrate surface

ABSTRACT

An object of the invention is to provide a method for removing foreign matter from a glass substrate surface to be finish-processed by a method accompanied with beam irradiation or laser light irradiation on the glass substrate surface. The present invention relates to a method for removing foreign matter from a glass substrate surface, which includes subjecting the glass substrate surface to gas cluster ion beam etching at an accelerating voltage of from 5 to 15 keV.

TECHNICAL FIELD

The present invention relates to a method for removing foreign matterfrom a glass substrate surface and, in particular, to a method forremoving foreign matter from a glass substrate surface which is requiredto have a high flatness, such as glass substrates to be used for areflective type mask for EUV (extreme ultraviolet) lithography in thesemiconductor manufacturing process. More specifically, the inventionrelates to a method for removing foreign matter from a glass substratesurface to be processed by a method accompanied with beam irradiation orlaser light irradiation on a glass substrate surface, such as ion beametching, gas cluster ion beam etching, plasma etching and nano-abrasionby means of laser light irradiation.

Also, the invention relates to a method comprising removing foreignmatter from a glass substrate surface, and then processing the glasssubstrate surface by a method accompanied with beam irradiation or laserlight irradiation on the glass substrate surface, such as ion beametching, gas cluster ion beam etching, plasma etching and nano-abrasionby means of laser light irradiation.

BACKGROUND ART

In the lithography technology, an exposure tool for manufacturing anintegrated circuit by transferring a fine circuit pattern onto a waferhas hitherto been widely utilized. With the trend toward higherintegration of an integrated circuit, higher speed and higherfunctionalization, the integrated circuit is becoming finer. For thatreason, the exposure tool is required to achieve image formation of acircuit pattern on a wafer surface with high resolution and a long focaldepth, and shortening of the wavelength of an exposure light source isbeing advanced. The exposure light source is further advancing fromconventional g-rays (wavelength: 436 nm), i-rays (wavelength: 365 nm)and a KrF excimer laser (wavelength: 248 nm), and an ArF excimer laser(wavelength: 193 nm) is started to be used. Also, in order to cope witha next-generation integrated circuit whose circuit line width willbecome not more than 100 nm, the use of an F₂ laser (wavelength: 157 nm)as an exposure light source is regarded as promising. However, it isconsidered that even this is able to cover only the generation with aline width of up to 70 nm.

Under such a technical trend, a lithography technology using EUV lightas a next-generation exposure light source is considered to beapplicable over plural generations after the line width of 45 nm and isattracting attention. The EUV light refers to light of a waveband of asoft X-ray region or vacuum ultraviolet region, and specifically meanslight having a wavelength of from about 0.2 to 100 nm. At present, theuse of 13.5 nm as a lithography light source is investigated. Theexposure principle of this EUV lithography (hereinafter abbreviated as“EUVL”) is identical with the conventional lithography in respect oftransferring a mask pattern using a projection optical system. However,in an energy region of EUV light, since there is no material capable oftransmitting the light therethrough, a refraction optical system cannotbe used, but a reflective optical system is inevitably used (see PatentDocument 1).

The mask to be used for EUVL is basically configured of (1) a glasssubstrate, (2) a reflective multilayer film formed on the glasssubstrate and (3) an absorber layer formed on the reflective multilayerfilm. As the reflective multilayer film, one having a structure in whichplural materials having a different refractive index against thewavelength of the exposure light are cyclically laminated at a nanometerscale is used, and Mo and Si are known as representative materials.Also, Ta and Cr has been investigated as the material for the absorberlayer. As the glass substrate, in order that a strain may not begenerated even upon irradiation with EUV light, a material having a lowheat expansion coefficient is required, and the use of a glass having alow heat expansion coefficient or a crystallized glass having a low heatexpansion coefficient has been investigated. In this specification, theglass having a low heat expansion coefficient and the crystallized glasshaving a low heat expansion coefficient are collectively called “lowexpansion glass” or “ultra-low expansion glass”.

As the low expansion glass or ultra-low expansion glass to be used as amask for EUVL, a quartz glass mainly composed of SiO₂ and to which TiO₂,SnO₂ or ZrO₂ is added as a dopant for the purpose of reducing the heatexpansion coefficient of glass is most widely used.

The glass substrate is manufactured by processing a raw material of sucha glass or crystallized glass in high precision and washing it. In thecase of processing the glass substrate, in general, the glass substratesurface is pre-polished at a relatively high processing rate until itcomes to have predetermined flatness and surface roughness; foreignmatter such as a polishing waste generated by pre-polishing is removedby washing; and the glass substrate surface is then finish-processed bya method with higher processing precision so as to have desired flatnessand surface roughness. As the method with high processing precision tobe used for finish-processing, a method accompanied with beamirradiation or laser light irradiation on a glass substrate surface,such as ion beam etching, gas cluster ion beam etching, plasma etchingand nano-abrasion by means of laser light irradiation, is preferablyused.

However, there are the case where foreign matter which has not beencompletely removed by washing remains; and the case where foreign matternewly attaches onto the glass substrate surface after washing. When theglass substrate surface where such foreign matter exists isfinish-processed by a method accompanied with beam irradiation or laserlight irradiation on a glass substrate surface, such as ion beametching, gas cluster ion beam etching, plasma etching and nano-abrasionby means of laser light irradiation, there is a problem that the portionon the glass substrate surface where foreign matter exists is notprocessed, and the subsequent washing to remove foreign matter from theglass substrate surface results in generation of convex defects of theglass.

Patent Document 1: JP-T-2003-505891

DISCLOSURE OF THE INVENTION

In order to solve of the foregoing problems of the related art, anobject of the invention is to provide a method for removing foreignmatter from a glass substrate surface to be finish-processed by a methodaccompanied with beam irradiation or laser light irradiation on theglass substrate surface, such as ion beam etching, gas cluster ion beametching, plasma etching and nano-abrasion by means of laser lightirradiation.

Another object of the invention is to provide a method in which afterremoving foreign matter from a glass substrate surface, the glasssubstrate surface is processed by a method accompanied with beamirradiation or laser light irradiation on a glass substrate surface,such as ion beam etching, gas cluster ion beam etching, plasma etchingand nano-abrasion by means of laser light irradiation.

In order to achieve the foregoing objects, the invention is to provide amethod for removing foreign matter from a glass substrate surface, whichcomprises subjecting the glass substrate surface to gas cluster ion beametching at an accelerating voltage of from 5 to 15 keV.

Also, the invention is to provide a method for removing foreign matterfrom a glass substrate surface, which comprises subjecting the glasssubstrate surface to gas cluster ion beam etching with, as a gas source,at least one gas selected from the group consisting of O₂, Ar, B, CO₂,N₂, N₂O and a boron hydride.

In this invention, each of the methods described in the preceding twoparagraphs is hereinafter referred to as “foreign matter removal methodof the invention”.

In the foreign matter removal method of the invention, it is preferablethat the gas cluster ion beam etching is performed under a conditionthat the etching amount is not more than 20 nm.

In the foreign matter removal method of the invention, it is preferablethat the glass substrate is made of a low expansion glass having a heatexpansion coefficient at 20° C. or at from 50 to 80° C. of 0±30 ppb/° C.

In the foreign matter removal method of the invention, it is preferablethat the glass substrate surface before performing the gas cluster ionbeam etching has a surface roughness (Rms) of not more than 5 nm.

In the foreign matter removal method of the invention, it is preferablethat the gas cluster ion beam etching is performed under a conditionthat a cluster size is 2,000 or more.

In the foreign matter removal method of the invention, it is preferablethat the gas cluster ion beam etching is performed while keeping anangle formed by a normal line of the glass substrate and a gas clusterion beam to be made incident to the glass substrate surface at from 3 to60 degrees.

Here, it is preferable that the gas cluster ion beam etching isperformed while keeping the glass substrate surface in a state facingdownward relative to the horizontal direction by from 3 to 60 degrees.

Also, the invention is to provide a method for processing a glasssubstrate surface, which comprises the steps of:

removing foreign matter on the glass substrate surface by the foreignmatter removal method of the invention; and

processing the glass substrate surface by a processing method selectedfrom the group consisting of ion beam etching, gas cluster ion beametching, plasma etching and nano-abrasion (this method will behereinafter referred to as “processing method (1) of the invention”).

In the processing method (1) of the invention, it is preferable that theprocessing method is gas cluster ion beam etching.

Here, it is preferable that the gas cluster ion beam etching in theprocessing step is performed at an accelerating voltage exceeding 15keV, using, as a source gas, a mixed gas selected from the groupconsisting of: a mixed gas of SF₆ and O₂; a mixed gas of SF₆, Ar and O₂;a mixed gas of NF₃ and O₂; a mixed gas of NF₃, Ar and O₂; a mixed gas ofNF₃ and N₂; and a mixed gas of NF₃, Ar and N₂.

It is preferable that the source gas is any one mixed gas selected fromthe group consisting of: a mixed gas of SF₆ and O₂; a mixed gas of SF₆,Ar and O₂; a mixed gas of NF₃ and O₂; and a mixed gas of NF₃, Ar and O₂.

Also, the invention is to provide a method for processing a glasssubstrate surface, which comprises the steps of:

measuring a flatness of the glass substrate surface;

removing foreign matter on the glass substrate surface by the foreignmatter removal method of the invention; and

processing the glass substrate surface by a processing method selectedfrom the group consisting of ion beam etching, gas cluster ion beametching, plasma etching and nano-abrasion,

wherein, in the step of processing the glass substrate surface, aprocessing condition of the glass substrate surface is set up for eachsite of the glass substrate on the basis of a result obtained from thestep of measuring a flatness (this method will be hereinafter referredto as “processing method (2) of the invention”).

In the processing method (2) of the invention, it is preferable that theprocessing method is ion beam etching, gas cluster ion beam etching orplasma etching; a width of waviness existing on the glass substratesurface is specified on the basis of a result obtained from the step ofmeasuring a flatness of the glass substrate surface; and the glasssubstrate surface is processed with a beam having a beam diameter of notmore than the width of the waviness in terms of FWHM (full width of halfmaximum) value.

Here, it is preferable that the FWHM value of the beam diameter is notmore than ½ of the width of the waviness.

In the processing method (2) of the invention, it is preferable that theprocessing method is gas cluster beam etching; and the gas cluster ionbeam etching in the processing step is performed at an acceleratingvoltage exceeding 15 keV, using, as a source gas, a mixed gas selectedfrom the group consisting of: any one mixed gas of SF₆ and O₂; a mixedgas of SF₆, Ar and O₂; a mixed gas of NF₃ and O₂; a mixed gas of NF₃, Arand O₂; a mixed gas of NF₃ and N₂; and a mixed gas of NF₃, Ar and N₂.

It is more preferable that the source gas is any one mixed gas selectedfrom the group consisting of: a mixed gas of SF₆ and O₂; a mixed gas ofSF₆, Ar and O₂; a mixed gas of NF₃ and O₂; and a mixed gas of NF₃, Arand O₂.

In the processing methods (1) and (2) of the invention, it is preferablethat subsequent to the step of processing the glass substrate surface, asecond processing step is performed for improving a surface roughness ofthe glass substrate surface.

It is preferable that as the second processing step, gas cluster ionbeam etching is performed at an accelerating voltage of 3 keV or moreand less than 30 keV, using, as a source gas, an O₂ gas singly or amixed gas of O₂ and at least one gas selected from the group consistingof Ar, CO and CO₂.

It is preferable that as the second processing step, mechanicalpolishing using a polishing slurry is performed at a surface pressure offrom 1 to 60 gf/cm².

Also, the invention is to provide a glass substrate obtained by theprocessing method of the invention, wherein the substrate surface has aflatness of not more than 50 nm, and is free from a convex glass defecthaving a height exceeding 1.5 nm.

According to the invention, in the case where a glass substrate surfaceis finish-processed by a method accompanied with beam irradiation orlaser light irradiation on the glass substrate surface, such as ion beametching, gas cluster ion beam etching, plasma etching and nano-abrasionby means of laser light irradiation, it is possible to prevent thegeneration of a convex defect of the glass on the glass substratesurface after processing and to process the glass substrate surface intoa surface having excellent flatness and surface roughness.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view illustrating the relationship between aprocessing surface of a substrate and GCIB in the foreign matter removalmethod of the invention.

The reference numerals used in the drawings denote the following,respectively.

1: Substrate

10: Processing surface

BEST MODE FOR CARRYING OUT THE INVENTION

The foreign matter removal method of the invention is concerned with amethod for removing foreign matter from a glass substrate surface to befinish-processed (this glass substrate surface will be also referred toas “processing surface”), namely the processing surface beforefinish-processing, by a method accompanied with beam irradiation orlaser light irradiation on a glass substrate surface, such as ion beametching, gas cluster ion beam etching, plasma etching and nano-abrasionby means of laser light irradiation.

Such a processing surface is washed before performing thefinish-processing. On that occasion, there is the case where foreignmatter which has not been completely removed by washing remains; or thecase where foreign matter newly attaches onto the glass substratesurface after washing. The foreign matter removal method of theinvention is aimed to remove such foreign matter.

The foreign matter which is a target of the foreign matter removalmethod of the invention refers to a material attaching onto theprocessing surface by a van der Waals force but not a material fixingonto the processing surface by a chemical bond, and its size is usuallyfrom about 1 to 2 μm or smaller.

The glass substrate which is a target of the foreign matter removalmethod of the invention is a glass substrate for a reflective type maskfor EUVL capable of mainly coping with higher integration and higherdefinition of an integrated circuit. The glass substrate to be used forthis application is a glass substrate having a small heat expansioncoefficient and a small scattering thereof. The glass substrate ispreferably made of a low expansion glass having a heat expansioncoefficient at 20° C. or at from 50 to 80° C. of 0±30 ppb/° C., and morepreferably made of an ultra-low expansion glass having a heat expansioncoefficient at 20° C. or at from 50 to 80° C. of 0±10 ppb/° C.

The glass substrate is not particularly limited with respect to theshape, size and thickness and the like. In the case of a substrate for areflective type mask for EUVL, its shape is a rectangular plate-shapedbody in view of a rectangular planar shape.

It is preferable that in the glass substrate which is a target of theforeign matter removal method of the invention, the processing surfaceis preliminarily processed so as to have predetermined flatness andsurface roughness.

Though ion beam etching, gas cluster ion beam etching, plasma etchingand nano-abrasion to be used for preliminary processing of theprocessing surface are able to process the glass substrate surface intoa surface with excellent flatness and surface roughness, theseprocessing methods are inferior to the conventional mechanical polishingin view of a processing rate, especially a processing rate in the caseof processing a glass substrate surface with a large area. On the otherhand, the foreign matter removal method of the invention, which isdescribed below in detail, is a method for removing foreign matterexisting on the processing surface without substantially processing theprocessing surface. For that reason, it is preferable that prior toperforming the foreign matter removal method of the invention, theprocessing surface is preliminarily processed so as to havepredetermined flatness and surface roughness by a processing methodhaving a relatively high processing rate.

The processing method to be used for the preliminary processing is notparticularly limited but can be widely chosen among known processingmethods to be used for processing a glass surface. However, by using apolishing pad having a large processing rate and a large surface area,it is possible to perform polishing processing with a large area atonce, and therefore, a mechanical polishing method is usually used. Themechanical polishing method as referred to herein includes, in additionto polishing processing only by means of a polishing function with anabrasive grain, a method of using a polishing slurry which utilizes apolishing function with an abrasive grain and a chemical polishingfunction with a chemical in combination. The mechanical polishing methodmay be any of lapping and polishing, and a polishing tool and anabrasive to be used can be appropriately chosen from known ones. Whenthe mechanical polishing method is used, for the purpose of making theprocessing rate large, it is preferable in the case of lapping that thelapping is performed at a surface pressure of from 30 to 70 gf/cm², andmore preferably at a surface pressure of from 40 to 60 gf/cm²; whereasit is preferable in the case of polishing that the polishing isperformed at a surface pressure of from 60 to 140 gf/cm², and morepreferably at a surface pressure of from 80 to 120 gf/cm². The lappingis preferably performed so as to give a lapping amount of from 100 to300 μm, and the polishing is preferably performed so as to give apolishing amount of from 1 to 60 μm.

In the case of performing preliminary processing, the processing surfaceafter the preliminary processing preferably has a surface roughness(Rms) of not more than 5 nm, and more preferably not more than 1 nm. Thesurface roughness as referred to in this specification means a surfaceroughness measured by an atomic force microscope with respect to an areaof from 1 to 10 μm square. When the surface roughness of the glasssubstrate after the preliminary processing exceeds 5 nm, it takes aconsiderably long period of time for finish-processing the processingsurface so as to have predetermined flatness and surface roughness afterperforming the foreign matter removal method of the invention, andtherefore, such becomes a factor in the increase of costs.

The foreign matter removal method of the invention is characterized inthat by performing gas cluster ion beam (hereinafter referred to as“GCIB”) etching under a specified condition that gives a low etchingamount with respect to the processing surface (this condition will behereinafter referred to as “low etching condition”), foreign matter isremoved from the processing surface while making the processing amountof the processing surface extremely low.

The GCIB etching as referred to herein is a method in which a reactivesubstance (source gas) which is in a gaseous state at normal temperatureand atmospheric pressure is jetted in a pressurized state into a vacuumapparatus via an expansion type nozzle to form a gas cluster, which isthen ionized upon irradiation with an electron, and the resulting GCIBis irradiated on a target to achieve etching. The gas cluster is usuallyconstituted of a massive atomic group or molecular group composed ofseveral thousand atoms or molecules. In the foreign matter removalmethod of the invention, by performing the GCIB etching on theprocessing surface, when the gas cluster comes into collision with theprocessing surface, a multiple collision effect is generated due to amutual action with the solid, whereby the foreign matter is removed fromthe processing surface. Then, since GCIB etching is performed on theprocessing surface under a low etching condition, the processing surfaceis not substantially processed.

In the foreign matter removal method of the invention, by specifying asite on the processing surface where foreign matter exists, the GCIBetching may be selectively performed in this site. However, it isdifficult to specify a site where a fine foreign matter having a size offrom about 1 to 2 μm or smaller and to selectively perform the GCIBetching in this site. Therefore, in general, the GCIB etching isperformed on the whole of the processing surface under a low etchingcondition. In that case, it is necessary that GCIB is scanned on theprocessing surface. As a method of scanning GCIB, luster scanning andspiral scanning are known, and any of these methods may be used.

In a first embodiment of the foreign matter removal method of theinvention, in order to perform GCIB etching on the processing surfaceunder a low etching condition, an accelerating voltage for applying toaccelerating electrodes is controlled at from 5 to 15 keV. In that case,the source gas which is used in performing the GCIB etching for thepurpose of finish-processing the glass substrate surface may be aconventionally used source gas. Specific examples of such a conventionalsource gas include SF₆, NF₃, CHF₃, CF₄, C₂F₆, C₃F₈, C₄F₆, SiF₄ and COF₂.These gases may be used singly or in admixture.

By controlling the accelerating voltage to be applied to acceleratingelectrodes at from 5 to 15 keV, even in the case where a conventionallyused source gas is used in performing the GCIB etching for the purposeof finish-processing the processing surface, the etching amount on theprocessing surface is sufficiently low, and foreign matter existing onthe processing surface can be removed without substantially processingthe processing surface. In the case where the accelerating voltage isless than 5 keV, when the gas cluster comes into collision with theprocessing surface, the kinetic energy is small so that foreign matterexisting on the processing surface cannot be removed depending upon thesize of the foreign matter. Specifically, foreign matter having a sizeof from about 1 to 2 μm cannot be removed. In the case where theaccelerating voltage exceeds 15 keV, when the gas cluster comes intocollision with the processing surface, the kinetic energy is large.Therefore, an etching action on the processing surface becomes moreremarkable than an action to remove a fine foreign matter existing onthe processing surface; and likewise the case where the processingsurface on which foreign matter exists is finish-processed by GCIBetching, only a portion of the processing surface where foreign matterexists remains without being etched, resulting in a problem that aconvex defect is generated on the processing surface from which foreignmatter has been removed by washing or the like.

The accelerating voltage is more preferably from 5 to 10 keV.

In a second embodiment of the foreign matter removal method of theinvention, in order to perform GCIB etching on the processing surfaceunder a low etching condition, the GCIB etching is performed using, as agas source, at least one gas selected from O₂, Ar, B, CO₂, N₂O and aboron hydride (for example, BH₃ and B₄H₁₀). When such a gas speciescomes into collision with the processing surface, it hardly causes achemical reaction, and an action to etch the processing surface isextremely weak. In the case where the GCIB etching is performed usingsuch a gas species as the source gas, foreign matter existing on theprocessing surface can be removed without substantially processing theprocessing surface.

In the second embodiment of the foreign matter removal method of theinvention, the accelerating voltage for applying a gas species having anextremely weak etching action to accelerating electrodes is notparticularly limited. However, what the accelerating voltage is 15 keVor more is preferable from the standpoint of removing foreign matterexisting on the processing surface without substantially processing theprocessing surface. The accelerating voltage is more preferably 20 keVor more, and further preferably 30 keV or more. In the case where theaccelerating voltage is less than 15 keV, when the gas cluster comesinto collision with the processing surface, the kinetic energy is small,and therefore, there is a possibility that foreign matter existing onthe processing surface cannot be removed depending upon the size of theforeign matter. However, even when the accelerating voltage is less than15 keV, by adjusting the cluster size, dose amount, irradiation time andthe like, foreign matter existing on the processing surface can beremoved.

According to the foregoing first embodiment and second embodiment of theforeign matter removal method of the invention, since GCIB etching isperformed on the processing surface under a low etching condition,foreign matter existing on the processing surface can be removed withoutsubstantially processing the processing surface. The low etchingcondition as referred to herein is preferably a condition under whichthe etching amount is not more than 20 nm, and more preferably acondition under which the etching amount is not more than 10 nm.

In the first embodiment and second embodiment of the foreign matterremoval method of the invention, the irradiation condition, for example,a cluster size, an ionizing current to be applied to ionizationelectrodes of a GCIB etching apparatus for ionizing the cluster and adose amount of GCIB can be appropriately chosen in accordance with thekind of a source gas, the accelerating voltage to be applied toaccelerating electrodes and the like. It is preferable that the GCIBetching is performed under a condition that a cluster size is 2,000 ormore. When the cluster size is 2,000 or more, since a relatively largegas cluster comes into collision with the processing surface, it isexpected that an effect for removing foreign matter existing on theprocessing surface is enhanced due to the multiple collision effect. Thecluster size is more preferably 3,000 or more, and especially preferably5,000 or more.

In the first embodiment and second embodiment of the foreign matterremoval method of the invention, it is preferable that GCIB isirradiated on the processing surface from an oblique direction thereto.When GCIB is irradiated on the processing surface from an obliquedirection thereto, it is expected that an effect for removing foreignmatter existing on the processing surface is enhanced due to themultiple collision effect. FIG. 1 is a view illustrating the state thatGCIB is irradiated on a processing surface from an oblique directionthereto. In FIG. 1, it is preferable that an angle θ formed by a normalline N of a glass substrate 1 (accordingly, a normal line N of aprocessing surface 10) and GCIB to be made incident to the processingsurface 10 is kept at from 3 to 60 degrees. By keeping the angle θ at 3degrees or more, it is expected that an effect for removing foreignmatter existing on the processing surface is enhanced due to themultiple collision effect. On the other hand, when the angle θ is keptexceeding 60 degrees, a spot shape of GCIB on the processing surfacebecomes elliptical conspicuously. Therefore, when the gas cluster comesinto collision with the processing surface, the kinetic energy isdispersed, whereby the effect for removing foreign matter existing onthe processing surface is reduced. Also, since the spot shape of GCIB onthe processing surface becomes elliptical conspicuously, there is apossibility that the flatness of the processing surface after performingthe GCIB etching is deteriorated.

The angle θ is kept more preferably at from 10 to 60 degrees, andfurther preferably at from 30 to 60 degrees.

In the case where GCIB is irradiated on the processing surface from anoblique direction thereto, as illustrated in FIG. 1, it is preferablethat GCIB is irradiated in a horizontal direction in a state that theprocessing surface 10 faces downward relative to the horizontaldirection by from 3 to 60 degrees. According to this, not only an effectfor removing foreign matter existing on the processing surface isenhanced due to the multiple collision effect, but it is possible toprevent the removed foreign matter from the reattachment onto theprocessing surface. The processing surface 10 is kept in a state facingdownward relative to the horizontal direction preferably by from 10 to60 degrees, and more preferably by from 30 to 60 degrees.

The processing method (1) of the invention includes a step of removingforeign matter on a glass substrate surface by the foregoing foreignmatter removal method of the invention (this step will be hereinafterreferred to as “foreign matter removal step”); and a step of processingthe glass substrate surface by a processing method selected from thegroup consisting of ion beam etching, GCIB etching, plasma etching andnano-abrasion (this step will be hereinafter referred to as “processingstep”).

In the case where the processing surface is pre-polished so as to havepredetermined flatness and surface roughness, after removing foreignmatter existing on the processing surface by the foregoing foreignmatter removal method of the invention, the processing surface isfinish-processed by a processing method selected from the groupconsisting of ion beam etching, GCIB etching, plasma etching andnano-abrasion.

In order to prevent the attachment of a new foreign matter onto theprocessing surface after the foreign matter removal step, it ispreferable that the foreign matter removal step and the processing stepare performed in the same chamber or performed in chambers placed sideby side in such a manner that the substrate can be conveyed withoutbeing discharged from the apparatus. In the case of using GCIB etchingin the processing step, it is preferable to use the same GCIB etchingapparatus in the foreign matter removal step and the processing step.

Of the foregoing processing methods, it is preferable to use GCIBetching because the surface can be processed so as to have a smallsurface roughness and excellent smoothness.

In the case of using the GCIB etching, a gas such as SF₆, Ar, O₂, N₂,NF₃, N₂O, CHF₃, CF₄, C₂F₆, C₃F₈, C₄F₆, SiF₄ and COF₂ can be used singlyor in admixture as a source gas. Of these, SF₆, NF₃, CHF₃, CF₄, C₂F₆,C₃F₈, C₄F₆, SiF₄ and COF₂ are excellent as the source gas from thestandpoint of a chemical reaction which occurs when the gas clustercomes into collision with the processing surface. Above all, mixed gasescontaining SF₆ or NF₃, specifically a mixed gas of SF₆ and O₂, a mixedgas of SF₆, Ar and O₂, a mixed gas of NF₃ and O₂, a mixed gas of NF₃, Arand O₂, a mixed gas of NF₃ and N₂ and a mixed gas of NF₃, Ar and N₂ arepreferable for reasons that the etching rate is high and that theprocessing tact is enhanced. In such a mixed gas, though a favorablemixing ratio of the respective components varies with a condition suchas an irradiation condition, the following are preferable.

SF₆/O₂=0.1 to 5%/95 to 99.9% (a mixed gas of SF₆ and O₂)

SF₆/Ar/O₂=0.1 to 5%/9.9 to 49.9%/50 to 90% (a mixed gas of SF₆, Ar andO₂)

NF₃/O₂=0.1 to 5%/95 to 99.9% (a mixed gas of NF₃ and O₂)

NF₃/Ar/O₂=0.1 to 5%/9.9 to 49.9%/50 to 90% (a mixed gas of NF₃, Ar andO₂)

NF₃/N₂=0.1 to 5%/95 to 99.9% (a mixed gas of NF₃ and N₂)

NF₃/Ar/N₂=0.1 to 5%/9.9 to 49.9%/50 to 90% (a mixed gas of NF₃, Ar andN₂)

Of these mixed gases, a mixed gas of SF₆ and O₂, a mixed gas of SF₆, Arand O₂, a mixed gas of NF₃ and O₂ and a mixed gas of NF₃, Ar and O₂ arepreferable.

The irradiation condition, for example, a cluster size, an ionizingcurrent to be applied to ionization electrodes of a GCIB etchingapparatus for ionizing the cluster and a dose amount of GCIB can beappropriately chosen in accordance with the kind of the source gas, thesurface properties of the processing surface, the purpose offinish-processing and the like. For example, in the case wherefinish-processing is performed for the purpose of improving the flatnessof the processing surface after the preliminary processing, it ispreferable that the accelerating voltage to be applied to acceleratingelectrodes exceeds 15 keV; and for the purpose of improving the flatnessof the processing surface without excessively deteriorating the surfaceroughness, it is preferable that the accelerating voltage exceeds 15 keVand is not more than 30 keV.

Also, in the processing step, in the case of using GCIB etching, it isnecessary that GCIB is scanned on the processing surface. As a method ofscanning GCIB, luster scanning and spiral scanning are known, and any ofthese methods may be used.

The processing method (2) of the invention includes a step of measuringa flatness of a glass substrate surface (this step will be hereinafterreferred to as “flatness measuring step”); a step of removing foreignmatter on the processing surface by the foregoing foreign matter removalmethod of the invention (this step will be hereinafter referred to as“foreign matter removal step”); and a step of processing the processingsurface by a processing method selected from the group consisting of ionbeam etching, GCIB etching, plasma etching and nano-abrasion (this stepwill be hereinafter referred to as “processing step”), wherein in theprocessing step, a processing condition of the processing surface is setup for each site of the processing surface on the basis of a resultobtained from the flatness measuring step.

In the case of performing preliminary processing and finish-processingby ion beam etching, GCIB etching, plasma etching or nano-abrasion forthe purpose of processing the processing surface of a glass substrate,for example, the processing surface of a glass substrate for a mask forEUVL, there may be the case where partial waviness exists on theprocessing surface after the preliminary processing. The waviness asreferred to herein means irregularities having a cycle of from 5 to 30mm among cyclic irregularities existing on the processing surface.

It is difficult to remove such waviness by means of thefinish-processing so as to have a desired flatness on the processingsurface. Also, there may be the case where the waviness generated in thepreliminary processing grows into larger waviness in the course offinish-processing.

The processing method (2) of the invention is a method in which suchwaviness generated on the processing surface after the preliminaryprocessing is removed, and the processing surface is finish-processedinto a surface with excellent flatness.

In the processing method (2) of the invention, for the purpose ofsetting up a processing condition of the processing surface for eachsite of the processing surface on the basis of a result obtained fromthe flatness measuring step, it would be better that the flatnessmeasuring step is performed prior to the processing step. The flatnessmeasuring step may be performed after the foreign matter removal step.However, in order to prevent the attachment of a new foreign matter ontothe processing surface after the foreign matter removal step, it ispreferable that the flatness measuring step is performed prior to theforeign matter removal step.

Also, in order to prevent the attachment of a new foreign matter ontothe processing surface after the foreign matter removal step, it ispreferable that the foreign matter removal step and the processing stepare performed in the same chamber or performed in chambers placed sideby side in such a manner that the substrate can be conveyed withoutbeing discharged from the apparatus. In the case of using GCIB etchingin the processing step, it is preferable to use the same GCIB etchingapparatus in the foreign matter removal step and the processing step.

In the flatness measuring step, the flatness in each site of theprocessing surface, namely a difference of altitude is measured.Accordingly, the result obtained from the flatness measuring stepbecomes a flatness map showing a difference of altitude in each site ofthe processing surface (this will be hereinafter referred to as“flatness map”).

The flatness in each site of the processing surface can be, for example,measured by a laser interference type flatness measuring device.However, it should not be construed that the invention is limitedthereto. The flatness map may be prepared using a measurement resultobtained by measuring a difference of altitude in each site of theprocessing surface using a laser displacement gauge, an ultrasonicdisplacement gauge or a contact type displacement gauge.

In the processing method (2) of the invention, after performing theflatness measuring step and the foreign matter removal step, theprocessing condition of the processing surface is set up for each siteof the processing surface on the basis of a result obtained from theflatness measuring step.

As described previously, the result obtained from the flatness measuringstep becomes a flatness map. In the case where the coordinates of theprocessing surface as a two-dimensional planar shape are defined as (x,y), the flatness map is expressed by S(x, y) (μm). The processing timeis expressed by T(x, y) (min). In the case where the processing rate isdefined as Y (μm/min), the relationship of these is expressed by thefollowing equation.

T(x, y)=S(x, y)/Y

Accordingly, in the case where the processing condition of theprocessing surface is set up for each site of the processing surface onthe basis of a result obtained from the flatness measuring step, theprocessing condition, specifically the processing time is set up foreach site of the processing surface according to the foregoing equation.

In the processing step, in the case of using a method accompanied withbeam irradiation onto the processing surface, specifically in the caseof using ion beam etching, GCIB etching or plasma etching, theprocessing condition of the processing surface can be set up for eachsite of the processing surface on the basis of a result obtained fromthe flatness measuring step. A setup procedure for this is hereunderspecifically described.

In the case of performing this setup procedure, the width of thewaviness existing on the processing surface is specified using a resultobtained from the flatness measuring step. The width of the waviness asreferred to herein means a length of a concave portion or a convexportion in the convex-concave shape existing cyclically on theprocessing surface. Accordingly, the width of the waviness is usually ½of a cycle of the width of the waviness. In the case where a pluralnumber of waviness having different cycles exit, the width of thewaviness having the smallest cycle is taken as the width of the wavinessexisting on the processing surface.

As described previously, the measurement result obtained from theflatness measuring step is a flatness map showing a difference ofaltitude in each site of the processing surface. Accordingly, it ispossible to easily specify the width of the waviness existing on theprocessing surface from the flatness map.

Ion beam etching, GCIB etching or plasma etching is performed with abeam having a beam diameter of not more than the width of the wavinesson the basis of the width of the waviness as specified in the foregoingprocedure. The beam diameter as referred to herein is based on FWHM(full width of half maximum) value. In this specification, when the beamdiameter is referred to, it means the FWHM value of the beam diameter.In the processing step, it is more preferable to use a beam having abeam diameter of not more than ½ of the width of the waviness. By usinga beam having a beam diameter of not more than the width of thewaviness, it is possible to irradiate a beam while concentrating on thewaviness existing on the processing surface and to effectively removethe waviness.

In the processing step, when a method accompanied with the beamirradiation on the processing surface is used, namely in the case ofusing ion beam etching, GCIB etching or plasma etching, it is necessarythat a beam is scanned on the processing surface. This is because inorder to set up a processing condition of the processing surface foreach site of the processing surface, it is required to make the range tobe irradiated with a beam at one time small as far as possible. Inparticular, in the case of using a beam having a beam diameter of notmore than the width of the waviness, it is necessary to scan theprocessing surface with the beam. As a method of scanning with a beam,luster scanning and spiral scanning are known, and any of these methodsmay be used.

Of the foregoing processing methods, it is preferable to use GCIBetching because the surface can be processed so as to have a smallsurface roughness and excellent smoothness.

In the case of using the GCIB etching, the source gas and irradiationcondition are the same as those described regarding the processingmethod (1) of the invention.

When the processing step according to the processing methods (1) or (2)of the invention is performed, there may be the case where the surfaceroughness of the processing surface is somewhat deteriorated dependingupon the properties of the processing surface or the irradiationcondition of a beam. Also, there may be the case where even when adesired flatness can be achieved in the foregoing processing step, thesurface cannot be processed so as to have a desired surface roughnessdepending upon specifications of the glass substrate. For that reason,it is preferable that subsequent to the foregoing processing step(hereinafter referred to as “first processing step”), a secondprocessing step for the purpose of improving the surface roughness ofthe processing surface is performed.

In the second processing step, GCIB etching can be used. In that case,the GCIB etching is performed by changing an irradiation condition suchas a source gas, an ionizing current and an accelerating voltage fromthose used in the GCIB etching to be used in the foreign matter removalmethod and the GCIB etching to be used in the first processing step.Specifically, the GCIB etching is performed under an irradiationcondition such that the etching amount is lower than that in the GCIBetching to be used in the first processing step. In comparison with theGCIB etching to be used in the first processing step, the GCIB etchingis performed under a more gentle condition using a lower ionizingcurrent or a lower accelerating voltage. More specifically, theaccelerating voltage is preferably 3 keV or more and less than 30 keV,and more preferably from 3 to 20 keV. Also, it is preferable to use, asa source gas, an O₂ gas singly or a mixed gas of O₂ and at least one gasselected from the group consisting of Ar, CO and CO₂ from the standpointthat when the source gas comes into collision with the processingsurface, it hardly causes a chemical reaction. Above all, it ispreferable to use a mixed gas of O₂ and Ar.

Also, in the second processing step, mechanical polishing using apolishing slurry, which is called touch polishing, can be performed at alow surface pressure of from 1 to 60 gf/cm². In the touch polishing, aglass substrate is set interposed between polishing plates each providedwith a polishing pad made of a non-woven fabric, a woven fabric or thelike, and the polishing plates are relatively rotated against the glasssubstrate while feeding a slurry adjusted so as to have predeterminedproperties, thereby polishing processing the processing surface at asurface pressure of from 1 to 60 gf/cm².

As the polishing pad, for example, BELLATRIX K7512, manufactured byKanebo, Ltd. is useful. As the polishing slurry, it is preferable to usea colloidal silica-containing polishing slurry; and it is morepreferable to use a polishing slurry containing colloidal silica havingan average primary particle size of not more than 50 nm and water andadjusted so as to have a pH in the range of from 0.5 to 4. The surfacepressure of polishing is from 1 to 60 gf/cm². When the surface pressureexceeds 60 gf/cm², it is impossible to process the processing surface toa desired surface roughness due to the generation of a scratch scar onthe substrate surface or the like. Also, there is a possibility that arotation load of the polishing plates becomes large. When the surfacepressure is less than 1 gf/cm², it takes a long period of time for theprocessing, and hence, such is not practically useful. Also, when thesurface pressure is less than 30 gf/cm², it takes a long period of timefor the processing. Therefore, it is preferable that after processing ata surface pressure of from 30 to 60 gf/cm² to some extent, the surfaceis finish-processed at a surface pressure of from 1 to 30 gf/cm².

The average primary particle size of colloidal silica is preferably lessthan 20 nm, more preferably less than 15 nm, and especially preferablyless than 10 nm. When the average primary particle size of colloidalsilica exceeds 50 nm, it is difficult to process the processing surfaceso as to have a desired surface roughness. Also, from the viewpoint ofpainstakingly managing the particle size, it is desirable that thecolloidal silica does not contain a secondary particle which is formedthrough coagulation of the primary particle as far as possible. In thecase where the colloidal silica contains a secondary particle, itsaverage particle size is preferably not more than 70 nm. The particlesize of colloidal silica as referred to herein is a particle sizeobtained by measuring an image with a magnification of from 15 to105×10³ times by SEM (scanning electron microscope).

The content of colloidal silica in the polishing slurry is preferablyfrom 10 to 30% by mass. When the content of colloidal silica in thepolishing slurry is less than 10% by mass, there is a possibility thatthe polishing efficiency may become worse, whereby economic polishing isnot attained. On the other hand, when the content of colloidal silicaexceeds 30% by mass, since the use amount of colloidal silica increases,there is a possibility of causing a trouble from the viewpoints of costsand washability. The content of colloidal silica in the polishing slurryis more preferably from 18 to 25% by mass, and especially preferablyfrom 18 to 22% by mass.

When the pH of the polishing slurry is made to fall within the foregoingacidic range, namely the pH is made to fall within the range of from 0.5to 4, it is possible to subject the processing surface to chemical andmechanical polishing processing, thereby achieving efficient polishingprocessing of the processing surface with good smoothness. That is, theconvex portions of the processing surface are softened by an acid of thepolishing slurry, and therefore, the convex portions can be easilyremoved by mechanical polishing. According to this, not only theprocessing efficiency is enhanced, but a glass waste which has beenremoved off by the polishing processing is softened, and therefore, thegeneration of a new damage due to the glass waste or the like isprevented. When the pH of the polishing slurry is less than 0.5, thereis a possibility that corrosion is generated in a polishing machine tobe used for touch polishing. From the viewpoint of handling propertiesof the polishing slurry, the pH is preferably 1 or more. In order toobtain a sufficient chemical polishing processing effect, the pH ispreferably not more than 4, and especially preferably in the range offrom 1.8 to 2.5.

The pH adjustment of the polishing slurry can be achieved by adding aninorganic acid or an organic acid singly or a combination thereof.Examples of the inorganic acid which can be used include nitric acid,sulfuric acid, hydrochloric acid, perchloric acid and phosphoric acid.Of these, nitric acid is preferable in view of easiness of handling.Also, examples of the organic acid include oxalic acid and citric acid.

As water to be used for the polishing slurry, pure water or ultrapurewater from which foreign matter has been removed is preferably used.That is, pure water or ultrapure water which substantially has not morethan one fine particle having a maximum size, as measured by a lightscattering mode using laser light or the like, of 0.1 μm or more per mLis preferable. When more than one foreign matter per mL is incorporatedregardless of the material quality or shape, there is a possibility thata surface defect such as a scratch and a pit is formed on the processingsurface. The foreign matter in pure water or ultrapure water can beremoved by, for example, filtration or ultrafiltration with a membranefilter, but it should not be construed that the removal method offoreign matter is limited thereto.

In the glass substrate as processed by the processing methods (1) or (2)of the invention, the processing surface has excellent flatness andsurface roughness; the flatness of the processing surface afterprocessing is not more than 50 nm; and a convex defect of the glasshaving a height exceeding 1.5 nm does not exist on the processingsurface. The flatness of the processing surface after processing is morepreferably not more than 30 nm, and further preferably not more than 20nm.

INDUSTRIAL APPLICABILITY

The glass substrate as processed by the processing method of theinvention is favorable as an optical device to be used in an opticalsystem of an exposure tool for semiconductor manufacture, especially anoptical device to be used in an optical system of a next-generationexposure tool for semiconductor manufacture with a line width of notmore than 45 nm because the processing surface has excellent flatnessand surface roughness. Specific examples of such an optical deviceinclude lenses, diffraction gratings, optical membrane bodies andcomposite bodies thereof, for example, lenses, multi-lenses, lensarrays, lenticular lenses, fly-eye lenses, aspheric lenses, mirrors,diffraction gratings, binary optics devices, photomasks and compositebodies thereof.

Also, the glass substrate as processed by the processing method of theinvention is favorable as a photomask and a mask blanks formanufacturing this photomask, especially a reflective type mask for EUVLand a mask blanks for manufacturing this mask.

The light source of the exposure tool is not particularly limited andmay be a conventional laser capable of emitting g-rays (wavelength: 436nm) or i-rays (wavelength: 365 nm). However, light sources of a shorterwavelength, specifically light sources having a wavelength of not morethan 250 nm are preferable. Specific examples of such a light sourceinclude a Kr F excimer laser (wavelength: 248 nm), an ArF excimer laser(wavelength: 193 nm), an F₂ laser (wavelength: 157 nm) and EUV(wavelength: 13.5 nm).

While the present invention has been described in detail and withreference to specific embodiments thereof, it will be apparent to oneskilled in the art that various changes and modifications can be madetherein without departing from the spirit and scope thereof.

This application is based on Japanese Patent Application No. 2007-172274filed Jun. 29, 2007, and the contents thereof are herein incorporated byreference.

1. A method for removing foreign matter from a glass substrate surface,which comprises subjecting the glass substrate surface to gas clusterion beam etching at an accelerating voltage of from 5 to 15 keV.
 2. Amethod for removing foreign matter from a glass substrate surface, whichcomprises subjecting the glass substrate surface to gas cluster ion beametching with, as a gas source, at least one gas selected from the groupconsisting of O₂, Ar, B, CO₂, N₂, N₂O and a boron hydride.
 3. The methodfor removing foreign matter from a glass substrate surface according toclaim 1, wherein the gas cluster ion beam etching is performed under acondition that the etching amount is not more than 20 nm.
 4. The methodfor removing foreign matter from a glass substrate surface according toclaim 1, wherein the glass substrate is made of a low expansion glasshaving a heat expansion coefficient at 20° C. of 0±30 ppb/° C.
 5. Themethod for removing foreign matter from a glass substrate surfaceaccording to claim 1, wherein the glass substrate surface beforeperforming the gas cluster ion beam etching has a surface roughness(Rms) of not more than 5 nm.
 6. The method for removing foreign matterfrom a glass substrate surface according to claim 1, wherein the gascluster ion beam etching is performed under a condition that a clustersize is 2,000 or more.
 7. The method for removing foreign matter from aglass substrate surface according to claim 1, wherein the gas clusterion beam etching is performed while keeping an angle formed by a normalline of the glass substrate and a gas cluster ion beam to be madeincident to the glass substrate surface at from 3 to 60 degrees.
 8. Themethod for removing foreign matter from a glass substrate surfaceaccording to claim 7, wherein the gas cluster ion beam etching isperformed while keeping the glass substrate surface in a state facingdownward relative to the horizontal direction by from 3 to 60 degrees.9. A method for processing a glass substrate surface, which comprisesthe steps of: removing foreign matter on the glass substrate surface bythe method according to claim 1; and processing the glass substratesurface by a processing method selected from the group consisting of ionbeam etching, gas cluster ion beam etching, plasma etching andnano-abrasion.
 10. The method for processing a glass substrate surfaceaccording to claim 9, wherein the processing method is gas cluster ionbeam etching.
 11. The method for processing a glass substrate surfaceaccording to claim 10, wherein the gas cluster ion beam etching in theprocessing step is performed at an accelerating voltage exceeding 15keV, using, as a source gas, a mixed gas selected from the groupconsisting of: a mixed gas of SF₆ and O₂; a mixed gas of SF₆, Ar and O₂;a mixed gas of NF₃ and O₂; a mixed gas of NF₃, Ar and O₂; a mixed gas ofNF₃ and N₂; and a mixed gas of NF₃, Ar and N₂.
 12. The method forprocessing a glass substrate surface according to claim 11, wherein thesource gas is any one mixed gas selected from the group consisting of: amixed gas of SF₆ and O₂; a mixed gas of SF₆, Ar and O₂; a mixed gas ofNF₃ and O₂; and a mixed gas of NF₃, Ar and O₂.
 13. A method forprocessing a glass substrate surface, which comprises the steps of:measuring a flatness of the glass substrate surface; removing foreignmatter on the glass substrate surface by the method according to claim1; and processing the glass substrate surface by a processing methodselected from the group consisting of ion beam etching, gas cluster ionbeam etching, plasma etching and nano-abrasion, wherein, in the step ofprocessing the glass substrate surface, a processing condition of theglass substrate surface is set up for each site of the glass substrateon the basis of a result obtained from the step of measuring a flatness.14. The method for processing a glass substrate surface according toclaim 13, wherein the processing method is ion beam etching, gas clusterion beam etching or plasma etching, wherein a width of waviness existingon the glass substrate surface is specified on the basis of a resultobtained from the step of measuring a flatness of the glass substratesurface, and wherein the glass substrate surface is processed with abeam having a beam diameter of not more than the width of the wavinessin terms of FWHM (full width of half maximum) value.
 15. The method forprocessing a glass substrate surface according to claim 14, wherein theFWHM value of the beam diameter is not more than ½ of the width of thewaviness.
 16. The method for processing a glass substrate surfaceaccording to claim 15, wherein the processing method is gas cluster beametching, and wherein the gas cluster ion beam etching in the processingstep is performed at an accelerating voltage exceeding 15 keV, using, asa source gas, any one mixed gas selected from the group consisting of: amixed gas of SF₆ and O₂; a mixed gas of SF₆, Ar and O₂; a mixed gas ofNF₃ and O₂; a mixed gas of NF₃, Ar and O₂; a mixed gas of NF₃ and N₂;and a mixed gas of NF₃, Ar and N₂.
 17. The method for processing a glasssubstrate surface according to claim 16, wherein the source gas is anyone mixed gas selected from the group consisting of: a mixed gas of SF₆and O₂; a mixed gas of SF₆, Ar and O₂; a mixed gas of NF₃ and O₂; and amixed gas of NF₃, Ar and O₂.
 18. The method for processing a glasssubstrate surface according to claim 9, further comprising subsequent tothe step of processing the glass substrate surface, a second processingstep for improving a surface roughness of the glass substrate surface.19. The method for processing a glass substrate surface according toclaim 18, wherein the second processing step comprises gas cluster ionbeam etching at an accelerating voltage of 3 keV or more and less than30 keV, using, as a source gas, an O₂ gas singly or a mixed gas of O₂and at least one gas selected from the group consisting of Ar, CO andCO₂.
 20. The method for processing a glass substrate surface accordingto claim 18, wherein the second processing step comprises mechanicalpolishing using a polishing slurry and performed at a surface pressureof from 1 to 60 gf/cm².